Integrated circuit fabrication may involve photolithographic processing. For example, photoresist may be provided over a layer that is to be patterned. The photoresist may then be exposed to actinic energy which has been patterned by passing it through a photomask. The patterned actinic radiation exposes selected regions of the photoresist, while leaving other regions unexposed. The exposure of the photoresist to the actinic radiation may alter the solubility of the photoresist in a developer solvent. The developer solvent may thus be utilized to selectively remove either the exposed regions or the unexposed regions, to thereby form a pattern of openings extending partially or wholly through the photoresist. After the pattern of openings is formed in the photoresist, the photoresist may be used as a mask during subsequent etching of the layer under the photoresist, and/or for other process steps (such as, for example, implanting of dopant into such layer).
Photomasks may be fabricated to include a device region and a non-device region. In many applications, the non-device region is composed of a peripheral border region encircling the device region. The device region is the region in which the patterns represent the desired pattern to be imparted to photoresist. The non-device region is the region used for grasping the photomask, and may be the region in which patterns may be formed for alignment structures, bar codes, and other purposes. The term “photomask” may have a couple of different meanings. Historically, the masks utilized to pattern radiation have been categorized as being either photomasks or reticles. The term “photomask” has traditionally been used to refer to masks which define a pattern for an entirety of a wafer, and the term “reticle” has traditionally been used to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting this disclosure and the claims that follow, the terms “reticle” and “photomask” are utilized interchangeably to refer to tools that pattern either a portion of a wafer, or an entirety of a wafer.
Various types of photomasks are known in the art. For example, one type of photomask includes a transparent plate covered with regions of a radiation blocking material, such as chromium, which is used to define the semiconductor feature pattern to be projected by the mask. Such masks are called binary masks, since radiation is completely blocked by the radiation blocking material and fully transmitted through the transparent plate in areas not covered by the radiation blocking material.
Due in part to limitations imposed by the wavelength of light or other actinic energy used to transfer the pattern, resolution can degrade at the edges of the patterns of binary photomasks. Such has led to the development of phase-shifting photomasks. The phase-shifting masks can increase the resolution of patterns by creating phase-shifting regions in transparent areas of the photomask. Standard phase-shifting photomasks are generally formed in one of two manners. In a first, transparent films of appropriate thickness are deposited and patterned over the desired transparent areas using a second level lithography and etch technique. In a second, vertical trenches are etched into the transparent substrate. In both instances, the edges between the phase-shifted and unshifted regions generally result in a transition between high and low refractive index regions. These types of photomasks include transmission areas on either side of a patterned opaque feature. One of these transmission areas transmits light 180° out of phase from the other of the transmission areas, and both may transmit approximately 100% of the incident radiation. Light diffracted underneath the opaque regions from the phase-shifted regions may be destructively canceled by light that is not phase-shifted to thereby create null or “dark” areas.
Another type of phase-shifting photomask is known as an “attenuated” or “half-tone” phase-shifting photomask. Such photomasks include both transparent and less transmissive regions. Actinic energy/radiation passing through a partially transmissive region of such a photomask generally lacks the energy to substantially affect an imaging layer exposed through the photomask. However, the partially transmissive regions of such photomasks are designed to shift passing radiation 180° relative to the radiation passing through the completely transmissive regions and, as a consequence, the radiation passing through the partially transmissive regions destructively interferes with radiation diffracting out from the edges of the completely transmissive regions. Photomasks have been proposed that use both binary features and attenuating phase-shift mask features in the device area.
As minimum device pitch falls below 100 nanometers (for example, where minimum feature size or minimum critical dimension falls below 50 nanometers), attenuated phase-shift photomasks may begin to lose contrast with specific wavelengths of actinic energy.